The present invention relates generally to optical fiber communications and more particularly to biasing control schemes for electro-optic modulators.
A typical optical communications system includes a transmitter, an optical channel (e.g., optical fiber), and a receiver. Within the transmitter, an electro-optic modulator is often used to modulate an optical carrier with the information to be transmitted. The modulated carrier is transmitted across the optical channel to the receiver, which retrieves the information from the carrier.
External modulators are commonly used in a variety of 1310 nm and 1550 nm networks to modulate the optical carrier with an information signal. Advantageously, external modulators provide essentially pure amplitude modulation with little or substantially no frequency chirp. However common external modulators are often inherently non-linear devices. As a result, external modulators are often operated in a mode which minimizes the non-linear effects of their transfer function, such as second and higher order harmonics. Typically a bias signal is applied to the electro-optic external modulator to establish an operating point, or bias point, to maintain linear operation of the modulator.
Recent advancements in modulator technology have resulted in modulators whose performance is relatively constant over time. However, the bias point of a typical external modulator may vary due to temperature variations, signal fluctuations, manufacturing tolerances, optical reflections and other environmental factors. If the proper bias point is not maintained, the modulator will exhibit stronger non-linearity, including the generation of even-order harmonics and the reduction of the signal strength in one of the outputs. This, in turn, may decrease the maximum dynamic range of the optical communications link. Therefore, optical communication systems that utilize external modulators typically include a control loop to optimize the bias point of the modulator.
In one common approach to controlling the bias point of an external modulator, two pilot tones at different frequencies f1 and f2 are applied to the electro-optic modulator. If the modulator is not operating at its half-power point, the modulator produces at its output among other terms, a distortion product fd located at the sum frequency (f1+f2) and difference frequency (f1xe2x88x92f2) of the two pilot tones. The magnitude of the distortion product is indicative of the error between the desired DC bias signal value and the actual DC bias signal value. As an example, if the distortion product is equal to zero, the modulator is currently operating at its half-power point and therefore no bias error exists. However, if second-order harmonic energy is present, a bias error exists and the bias signal value is adjusted to null the bias error.
Even though the modulator is properly biased to eliminate even order distortions, it is likely that modulator performance will suffer from large odd order distortions. Since the pilot tones are typically not being corrected for odd order distortion products, for example through pre-distortion circuitry, odd order distortion products that fall within the frequency band of interest may occur. Third order distortion products typically will have a magnitude roughly equal to the magnitude of the pilot tones. A typical manifestation of third order distortion products appears as sidebands on each RF carrier. For example, referring to FIG. 1, for purposes of illustration assume an optical transmission system operates with one RF carrier, f1=200 MHz, and two pilot tones, fa=300 kHz and fb=400 kHz. Third order distortion products may therefore be produced at frequencies equal to f1+fa+fb and f1xe2x88x92fb, corresponding to 200.7 MHz and 199.3 MHz, respectively in this example.
In practice the pilot tones may be introduced into the modulator by injection through a bias port or an RF input port. Several design issues are created when the pilot tones are combined with the RF carriers and injected into the RF port. First, pilot tones are often relatively low frequency signals. This is typically the case since higher frequency bands are frequently allocated for other applications (e.g., 5 MHz-42 MHz for analog return path applications). In addition, low frequency circuit components with acceptable linearity and noise performance are readily available.
The tolerance of the frequency response of the RF drive circuitry is typically on the order of about +/xe2x88x920.75 dB for common optical communication systems. If the pilot tones are combined with the RF carriers the bandwidth of the frequency response of the RF drive circuitry may be greatly increased. A typical bandwidth may range from an upper end of the band (typically from 745 MHz to 1 GHz) down to the lower end of the band as defined by the pilot tone frequencies. Such wide bandwidth may result in a compromise in the RF design. In addition, to avoid the third order distortions described above, the predistortion circuitry must also operate down to the pilot tone frequencies. This can also impose constraints on the RF design.
When the pilot tones are applied to the modulator along with the DC bias through the bias port, the constraints on the frequency response and the pre-distortion mentioned above are removed. However, since the RF and the DC ports both operate on the same optical waveguides, the third order distortions shown in FIG. 1 may still occur. Third order distortions may be reduced by reducing the amplitude of the pilot tones. However, the amplitude of the pilot tones may not be reduced below the dynamic range of the detection electronics of the control loop. Conventionally, as the optimum half-power bias point is approached, the magnitude of the distortion product drops off relatively quickly making detection and processing of the distortion product difficult. In addition, typical transmitters often operate with the amplitude of the pilot tones very close to the signal to noise limits so that the pilot tones do not take away from the overall modulation depth of the modulator. Therefore, in many systems it may not be possible to substantially reduced the amplitude of the pilot tones to reduce third order distortion.
Another method to reduce third order distortion created when pilot tones are injected into the bias port of a modulator is to pulse the tones in the time domain and detect them synchronously with the control loop. However, burst mode synchronous control loops are difficult to design and operate.
Thus, there is a need for approaches to controlling the bias point of electro-optic modulators, such as Mach-Zehnder modulators, that does not introduce significant third order distortion products onto the transmitted optical signal.
In one aspect of the present invention, a bias control system for automatically controlling a bias point of an electro-optic modulator includes a pilot tone generator that generates a first pilot tone at a first frequency and a second pilot tone at a second frequency. The first and second tones are swept in frequency over a predetermined frequency range with a predetermined sweep rate, to spectrally spread third order distortion products over a larger frequency band, allowing the amplitude of the pilot tones to be increased and thereby increasing the gain of the bias control circuit.
The bias control system preferably further includes an optical detector for sampling the output of the electro-optic modulator, wherein the sampled optical output includes a distortion product resulting from the first and second pilot tones, and a feedback control circuit coupled to the optical detector and to the pilot tone generator for generating an error signal based on the distortion product in the sampled optical output for controlling the bias point of the modulator.
In another aspect of the invention, the difference between the frequency of the first pilot tone and the frequency of the second pilot tone is held constant over the entire range of excursion frequencies.
In an alternate embodiment, a method of automatically controlling a bias point of an electro-optic modulator includes generating a first pilot tone at a first frequency, generating a second pilot tone at a second frequency, sweeping the frequency of the first and second pilot tones over a predetermined frequency range at a predetermined sweep rate, detecting a distortion product in output of the modulator resulting from the first and second pilot tones, and generating an error signal as a function of the distortion product for controlling bias point of the modulator.